1. Field of the Invention
The present invention relates to a rangefinder apparatus and a camera equipped therewith.
2. Related Background Art
A rangefinder apparatus of a camera equipped with an AF sensor of passive type or the like captures an object by a pair of left and right line sensors, for example, so as to acquire left and right sensor images (AF data). A pair of window areas for acquiring a pair of AF data items used for correlation value computing from the AF data obtained by the pair of left and right line sensors are determined, and pairs of AF data items used for correlation value computing are acquired while shifting the pair of window areas in directions opposite from each other within a pair of predetermined sensor regions (employed sensors). Alternatively, one of the window areas is fixed, and pairs of AF data items used for correlation value computing are successively acquired while shifting the other window area. The correlation between thus obtained pair of AF data items is determined, and the object distance is calculated according to the shift amount of window areas at the time of yielding the highest correlation (at the time when the left and right sensor images in the respective employed sensors coincide with each other).
Specifically, the shift amount of window areas yielding the highest correlation is a shift amount at which the correlation value attains a minimum value. Though there may be cases where the highest correlation is obtained when the correlation value is at a maximum value depending on the method of computing correlation values, this specification explains a case where the highest correlation is obtained when the correlation value is at a minimum value. There are cases where a plurality of local minimum values exist. In such a case, the shift amount yielding the absolute minimum value is usually determined to be the shift amount with the highest correlation.
While conventional rangefinder apparatus of passive type perform correlation computing, and employ the shift amount yielding the highest correlation, a plurality of local minimum values exist in the case of an object to be subjected to rangefinding having a periodic pattern. In this case, a local minimum value other than one that should be adopted for determining the real distance to the object may be adopted. Therefore, it has conventionally been checked whether the local minimum value is reliable or not, so as not to choose the local minimum value without reliability.
For example, a rangefinder apparatus disclosed in Japanese Patent No. 3239413 checks the reliability of the local minimum value in the following way. This rangefinder apparatus starts shifting from the longer distance shift side and successively acquires correlation values. When a local minimum value appears, this rangefinder apparatus checks the reliability of the local minimum value by estimating the smallness (highness of correlation) of the local minimum value and the sharpness of the valley formed by the local minimum value. In this reliability checking process, threshold values for estimating the smallness and sharpness are varied so as to become stricter on the shorter side, thereby avoiding erroneous rangefinding which shifts to a shorter distance when measured at a longer distance.